The invention concerns carbon molecular sieve membranes (“CMS membranes”), and more particularly the use of such membranes in gas separation. In particular, the present disclosure concerns an advantageous method for producing CMS membranes with desired selectivity and permeability properties. It has been discovered that by controlling and selecting the oxygen concentration in the pyrolysis atmosphere used to produce CMS membranes, membrane selectivity and permeability can be adjusted. In particular, embodiments of the invention include optimizing acid gas permeability or selectivity by varying the oxygen concentration in the pyrolysis atmosphere. Further embodiments of the invention include using a combination of oxygen concentration and pyrolysis temperature to tune CMS performance.
Membranes are widely used for the separation of gases and liquids, including for example, separating acid gases, such as CO2 and H2S from natural gas, and the removal of O2 from air. Gas transport through such membranes is commonly modeled by the sorption-diffusion mechanism. Specifically, gas molecules sorb into the membrane at the upstream, diffuse through the membrane, and finally desorb from the membrane at the downstream. Two intrinsic properties are commonly used to evaluate the performance of a membrane material; “permeability” and “selectivity.” Permeability is hereby defined as a measure of the intrinsic productivity of a membrane material; more specifically, it is the partial pressure and thickness normalized flux, typically measured in Barrer. Selectivity, meanwhile, is a measure of the ability of one gas to permeate through the membrane versus a different gas; for example, the permeability of CO2 versus CH4, measured as a unit-less ratio.
Currently, polymeric membranes are well studied and widely available for gaseous separations due to easy processability and low cost. CMS membranes, however, have been shown to have attractive separation performance properties exceeding that of polymeric membranes.
CMS membranes are typically produced through thermal pyrolysis of polymer precursors. For example, it is known that defect-free hollow fiber CMS membranes can be produced by pyrolyzing cellulose hollow fibers (J. E. Koresh and A. Soffer, Molecular sieve permselective membrane. Part I. Presentation of a new device for gas mixture separation. Separation Science and Technology, 18, 8 (1983)). In addition, many other polymers have been used to produce CMS membranes in fiber and dense film form, among which polyimides have been favored. Polyimides have a high glass transition temperature, are easy to process, and have one of the highest separation performance properties among other polymeric membranes, even prior to pyrolysis.
U.S. Pat. No. 6,565,631 to Koros et al., which is incorporated herein by reference, describes a method of synthesizing CMS membranes. In particular, a polyimide hollow fiber was placed in a pyrolysis furnace with an evacuated environment, with a pyrolysis pressure of between 0.01 and 0.10 mm Hg air. U.S. Pat. No. 6,565,631 also discloses a method of using CMS membranes to separate CO2 from a methane stream containing 10% CO2, at 1000 psia and 50° C., with a selectivity of approximately 45, a selectivity that is much higher than typical commercial polymeric membranes. U.S. Pat. No. 6,565,631 also discloses that CMS membranes can, unlike polymeric membranes, operate with trace amounts of hydrocarbon impurities with little loss in selectivity. Other patents that describe processes for producing carbon membranes (both asymmetric hollow “filamentary” and flat sheets), and applications for gas separation, include U.S. Pat. No. 5,288,304, and EP Patent No. 459,623, which are incorporated herein in their entireties.
Prior research has shown that CMS membrane separation properties are primarily affected by the following factors: (1) pyrolysis precursor, (2) pyrolysis temperature, (3) thermal soak time, and (4) pyrolysis atmosphere. The first three factors have been investigated in detail, but the effect of the fourth factor, pyrolysis atmosphere, has remained largely unknown.
For example, Steel and Koros performed a detailed investigation of the impact of pyrolysis temperature, thermal soak time, and polymer composition on the performance of carbon membranes. (K. M. Steel and W. J. Koros, Investigation of Porosity of Carbon Materials and Related Effects on Gas Separation Properties, Carbon, 41, 253 (2003).) Membranes were produced in an air atmosphere at 0.05 mm Hg pressure. The results showed that increases in both temperature and thermal soak time increased the selectivity but decreased permeance for CO2/CH4 separation. In addition, Steel et al showed that a precursor polymer with a rigid, tightly packed structure tends to lead to a CMS membrane having higher selectivity compared with less rigid precursor polymers.
The impact of pyrolysis atmosphere has been researched only to a limited extent. Suda and Haraya disclosed the formation of CMS membranes under different environments. (H. Suda and K. Haraya, Gas Permeation Through Micropores of Carbon Molecular Sieve Membranes Derived From Kapton Polyimide, J. Phys. Chem. B, 101, 3988 (1997).) CMS dense films were prepared from polyimide Kapton® at 1000° C. in either argon or in vacuum. According to their gas separation properties, the results of an O2/N2 separation were almost the same between 6 membranes formed under the different atmospheres. Suda and Haraya did not disclose the effects of atmosphere on CO2 separation from natural gas, nor disclose how separation properties vary with oxygen concentration. Similarly, Geiszler and Koros disclosed the results of CMS fibers produced from pyrolysis of fluorinated polyimide in helium and argon for both O2/N2 and H2/N2 separations. (V. C. Geiszler and W. J. Koros, Effects of Polyimide Pyrolysis Atmosphere on Separation Performance of Carbon Molecular Sieve Membranes, J. Memb. Sci., (2009).). That paper disclosed a slightly higher selectivity and lower permeability with vacuum pyrolysis than the purged pyrolysis processes. In addition, Geiszler and Koros showed that the flow rate of the purge gases impacted performance. Geiszler and Koros, however, did not disclose the effects of atmosphere on CO2 separation from natural gas, or the effects of oxygen concentration on separation properties.
The present inventors recently extended the study of pyrolysis environments, and proposed in publications that a critical factor impacting the separation performance of CMS membranes is oxygen exposure during pyrolysis. (M. Kiyono, P. J. Williams, and W. J. Koros, Effect of Pyrolysis Atmosphere on Separation Performance of Carbon Molecular Sieve Membranes, J. Memb. Sci., (2009); P. J. Williams, Analysis of Factors Influencing the Performance of CMS Membrane for Gas Separation, Georgia Institute of Technology (2006).) In the paper by Kiyono et al., the pyrolysis environment was purged with specialty gases containing controlled amounts of oxygen ranging from 4-50 ppm O2 in argon flowing at 200 cc(STP)/min. The performance of the membranes was found to be a strong function of oxygen exposure. Neither paper, however, disclosed the effect of oxygen concentration, rather the total oxygen amount, on separation properties. The present inventors have since discovered that oxygen concentration, and not total oxygen amount, impacts the overall separation performance of the membranes and allows the membrane performance to be modified to optimize selectivity and permeability.